FIELD OF THE INVENTION
The present invention relates to a magnetically suspended centrifugal pump apparatus. More specifically, the present invention relates to a magnetically suspended centrifugal pump apparatus used for a medical equipment such as a blood pump, an artificial heart-lung and an artificial heart.
FIGS. 3A to 3C show cross sectional structure of a magnetically suspended centrifugal blood pump, in which FIG. 3B is a cross section taken along the line A--A of FIG. 3A and FIG. 3C is taken along the line B--B of FIG. 3A. The blood pump shown in FIG. 3A includes, as four main components, a magnetic bearing 1, a casing 2, an impeller 3 and a motor 4. As impeller 3 rotates, blood enters a central inlet port 5 of magnetic bearing 1, is pumped in casing 2, and discharged from an outlet port 6.
Impeller 3 includes, as shown in FIG. 4, permanent magnets 8 arranged spaced by an equal distance from each other circumferentially, and a ferromagnetic disc 9, provided on opposing sides of a vane 7. Impeller 3 floats in casing 2 on the balance between attractions on internal permanent magnets 8 and permanent magnets 10 arranged opposing thereto, or attractions between ferromagnetic disc 9 of impeller 3 on the opposite side and a plurality of magnetic bearings 1. At this time, impeller 3 is actively controlled by magnetic bearing 1. Control in the radial direction and driving for rotation are performed by a magnetic coupling provided by permanent magnets 8 and 10. Magnetic bearing 1 includes electromagnets 11 and a sensor 12. By calculating an output from sensor 12, current in each electromagnet 11 is controlled.
FIG. 5 shows a magnetic bearing control apparatus with respect to one axis only, in which movement in the directions of two axes, that is, .theta..sub.x and .theta..sub.y are not considered but only the movement and its control along the z axis are represented, facilitating understanding of the operation of magnetic bearing 1. Referring to FIG. 5, when control circuit 16 is in operation, position of impeller 3 is measured by a sensor 12, a signal is passed to electromagnet 11 through sensor amplifier 18, control circuit 16 and amplifier 17. In this manner, impeller 3 is controlled such that it floats at a prescribed position in casing 2, dependent on the balance between attraction of electromagnets 11 and attraction of magnetic coupling. The prescribed position is determined by a neutral position setting operation in advance, such that the position is at the center of casing 2 (where spaces between opposing surfaces of impeller 3 and housing are A=B), by adjusting sensor amplifier 18.
FIGS. 6 to 8 are illustrations showing the neutral position setting operation. In the neutral position setting operation, as a preparation, motor 4 shown in FIG. 5 is removed so that external force is not applied to impeller 3, as shown in FIG. 6, and current of electromagnets 11 are not provided. Direction and amount of movement of impeller 3 are represented by polarity and voltage of an output of sensor amplifier 18 from the neutral position. For this purpose, a spacer 23 having the thickness A which is the same as the space A at the end surface when impeller 3 is at the neutral position is placed between magnetic bearing 1 and impeller 3, so that impeller 3 is positioned at the neutral position, the output from sensor amplifier 18 at this time is set to 0 [v], and offset is adjusted.
Thereafter, referring to FIG. 7, in order to set an output gain with respect to the amount of movement, spacer 23 is removed, magnetic bearing 1 and impeller 3 are brought into tight contact, an output at that time is set to a predetermined voltage X [v], and the sensor gain is adjusted. As a result, when motor 4 removed from adjustment is again attached to casing 2 as shown in FIG. 8, impeller 3 would be brought into tight contact with that end surface of the casing which faces the magnetic coupling because of attraction by the magnetic coupling, when electromagnets 11 are not controlled. The output from sensor amplifier 18 is -X [v] at this time, since impeller 3 is at the same distance A in the opposite direction from the neutral position.
As described above, sensor offset adjustment is necessary when the impeller is exchanged, because of variations of ferromagnetic disc 9 used in impeller 3, as shown in FIG. 4. As a result, in a blood pump for open heart surgery in which impeller 3 is an article of consumption, it becomes necessary to perform the above described neutral position setting operation every time impeller 3 is exchanged, which requires labor for exchanging operation and skill for setting operation.